Energy and CO2 balances in different power generation routes using wood fuel from short rotation coppice

PII:

Biomass and Bioenergy Vol. 15, Nos 4/5, pp. 379±390, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0961-9534/98 $ - see front matter S0961-9534(98)00044-0

ENERGY AND CO2 BALANCES IN DIFFERENT POWER GENERATION ROUTES USING WOOD FUEL FROM SHORT ROTATION COPPICE XAVIER DUBUISSON* and IVAN SINTZOFF$ *Universite Catholique de Louvain, Laboratoire ECOP-Grandes Cultures, Place Croix du Sud 2/11, 1348 Louvain-la-Neuve, Belgium $Universite Catholique de Louvain, Groupe eÂnergie Biomasse, Place du Levant 2, 1348 Louvain-laNeuve, Belgium AbstractÐBy substituting fossil fuels and storing carbon in biomass and soil, the development of bioenergy can play a signi®cant role in the reduction of CO2 emissions to the atmosphere. The objective of this study was to carry out an energy and carbon analysis of di€erent power generation routes using wood fuel from short rotation coppice. Three scenarios of wood fuel production were considered, based on the level of intensi®cation of cultivation practices, in terms of machinery and materials input. Local and regional transportation were distinguished, as well as natural convection or forced ventilation for drying. We also studied three conversion systems: local peak electricity generation, local cogeneration of heat and power (CHP), and centralised power generation by wood and coal pulverisation co®ring. The energy and carbon balances of di€erent wood-energy routes were estimated by calculating direct and indirect energy and carbon costs of all their components (fuel, materials and machinery). Energy ratios of 22, 23 and 26 after storage and drying at the farm were obtained respectively for the di€erent scenarios. An average of 1.7 kg of carbon (kgC) is released per GJ of wood energy produced. Crop maintenance, and chemical fertilisation, account for 40% of total energy costs and 25% of total carbon costs of wood fuel production. Final avoided carbon emissions by fossil fuel substitution reach 6.3 to 8.8 tC haÿ1 yrÿ1 with CHP, 3.8 to 5.4 with peak power production and 3.3 to 4.6 with centralised co®ring. # 1998 Elsevier Science Ltd. All rights reserved KeywordsÐShort rotation coppice; wood energy; energy & carbon balance; power generation.

1. INTRODUCTION

alternative crop for agricultural diversi®cation, and create jobs in rural areas.

1.1. Background The use of fossil fuels is widely recognised as having a major negative impact on the global environment by the build up of greenhouse gases and the acidi®cation of soil and lakes.1 By substituting fossil fuels and storing carbon in biomass and soil, the development of bioenergy can play a signi®cant role in the reduction of CO2 emissions to the atmosphere. After the United Nations Conference on the Environment and Development (Rio, June 1992), Belgium launched a number of biomass initiatives aimed at developing a rich and varied potential (biofuels, biomethanisation, wood gasi®cation and combustion). Short rotation coppice (SRC) is gaining a lot of interest as an energy crop in Belgium due to its high yielding capacity and multiple environmental bene®t (CO2 mitigation, vegetation ®lter, low inputs, etc.). The production of SRC for energy generation should also provide an 379

1.2. Objective and approach of the study The objective of this study is to evaluate the energy performances, carbon balances and reduction of carbon emissions of generating power from short rotation coppice, taking into consideration the whole energy production system. The energy and carbon budgets were calculated for all the activities involved in cultivating SRC, for three di€erent scenarios (low, medium, high) based on the level of intensi®cation (mechanisation and inputs). We also distinguished natural and forced drying, local and regional transport. Three conversion systems for power generation were analysed: local electricity generation by gasi®cation, cogeneration of heat and power by gasi®cation, and wood and coal co®ring in a classical power plant. Avoided carbon emissions have been calculated for each ®nal energy conversion system

380

XAVIER DUBUISSON and IVAN SINTZOFF

by comparing with current electricity generation systems using fossil fuels. 1.3. Methodology A classical energy analysis was applied as described by Boustead and Hancock.2 Energy conversion systems were decomposed in a succession of transformation steps. Each step is seen as a system fed by di€erent inputs and producing wood or energy outputs. The energy budget for any given system refers to the direct and indirect inputs to that system which produce a speci®ed unit of system output. We estimated the fuel, materials and machinery inputs for all the activities of the wood-energy routes and calculated their direct and indirect energy and carbon costs. Energy and carbon costs of fossil fuels inputs were chosen consistent with European average supply, taking into account crude oil extraction, ocean or pipe-line transport, re®ning and distribution. Energy costs embodied in materials (fertilisers, buildings) and machinery were compiled from the literature.10,13,14,17±21 Final carbon emissions were compared with current available energy systems in Belgium. Avoided carbon emissions result from the substitution of fossil energy technologies in use in Belgium for new wood-energy systems. Unlike fossil fuels, the wood carbon content released to the atmosphere after energy oxidation is not considered as net carbon emissions. In fact, this ligno-cellulosic carbon has been ®xed from the atmosphere during the wood growth. We calculated the overall energy systems eciencies by dividing ®nal energy outputs by total energy inputs. 2. WOOD FUEL PRODUCTION

The energy and carbon budgets were calculated for all the activities of cultivating SRC, from ground preparation till harvest, as well as wood fuel storage and transport. The scenarios ``low'', ``medium'' and ``high'' represent three levels of intensi®cation of SRC cultivation with regard to the nature and the amount of inputs (fertilisers, pesticides, fence, etc.), as well as the scale of mechanisation. They were de®ned according to the standards of three typical farms in Belgium, looking at the machinery available, the scale and the nature of the farming system, and likely practices and land used for growing SRC. A typical farm of

the scenario ``low'', for example, could be a small farm (15 to 20 ha) in the Ardennes (hilly region in the South of Belgium) where SRC would be cultivated mainly on grasslands. 2.1. Inventory of the operations of wood fuel production We established an inventory of the di€erent operations of wood fuel production from our experience of SRC cultivation in Belgium, and the recommendations given in the literature.3±5 Table 1 presents a summary of the crop management and processing operations carried out, the machinery used and the inputs quantities for each scenario. In the scenario ``low'', SRC cultivation practices are relatively extensive. There is less care in crop management and a lower fertilisation than in other scenarios. A tractor of 80 hp is used to carry out most of the operations, and the machinery used is adapted to such power. In this scenario, SRC is more likely to be implanted on less fertile soils and the yields were estimated to be 15% lower than in the scenario ``medium'', i.e. 10 odt haÿ1 yrÿ1. The cultivated wood is harvested in sticks which are then allowed to dry by natural convection at the border of the ®eld during one season. They are then chipped by a tractor-mounted chipper in the ®eld. In the scenario ``medium'', cultivation practices are more intensive and the fertilisation is higher. An electrical fence is implanted at crop establishment to protect the plantation against browsing by mammals. The plantation is operated with the same semi-automated horticultural planter as in scenario ``low''. Harvest is carried out with a tractor-mounted chips harvester. Yields were estimated to be 12 odt haÿ1 yrÿ1, corresponding to average yields cited in the literature.3±5 In the scenario ``high'', crop management is more intensive than in the scenario ``medium'' and SRC is planted on good agricultural land. The yields we chose are high, 15 odt haÿ1 yrÿ1, and correspond to the results we obtained in our experiments on fertile loamy soils in Belgium.6 The plantation is operated with an automatic planter and harvest is carried out with an adapted self-propelled forage harvester. A permanent physical fence is implanted to protect the plantation. After harvest, we considered a local transport (5 km) to carry the wood fuel in chips

Generation routes using wood fule

from the ®eld to the farm, using a tractor with a trailer. In the scenario ``low'', the wood fuel has the required moisture content to enter in the conversion system (25%) after drying naturally in the ®eld, with a dry matter loss of 1% (estimation based on our measurements). In the scenario ``medium'' and ``high'', the harvested chips (50% moisture content) are stored and dried by intermittent forced ventilation during 31 weeks. The storage occurs at the farm under cover during 31 weeks after which the chips moisture content is 22%. A dry matter loss of 6%, due to respiratory selfheating during storage, is taken into account.7 We considered that storage would take place

381

in a covered area already available at the farm. If the energy conversion of the woodfuel takes place in a central plant, the chips have to be transported by truck from the farm to the plant (30 km). In each scenario, the level of fertiliser input was ®xed equal to the amount of nutrients that is exported by the harvested woody biomass.4 We assume a level of fertilisation sucient to maintain the initial fertility of the soil. The type and quantity of herbicides were chosen according to the results of our weed control experiments, which showed that a cocktail of simazine (1 kg haÿ1) and isoxaben (0.4 kg haÿ1) was the most ecient in our conditions.6

Table 1. Inventory of the operations of SRC cultivation Scenario ``low'' Tractor Ground preparation Subsoiling Ploughing Fencing Harrowing Crop establishment Planting Beat up Chemical weeding Mechanical weeding Cut back

112 kW (150 hp)

3 ploughshares Cultivator 5.5 m

4 ploughshares Electrical fence Power harrow 2.5 m

Subsoiler 3 teeth 5 ploughshares Physical fence Power harrow 3 m

Semi-automated, 2 rows, Catkin Manual, 3% failure Sprayer 15 m AZ500 0.8 kg/ha Premazin 2.0 kg/ha Static hoe, 2 m Brushcutter, 3.8 hp

Semi-automated, 2 rows Catkin Manual, 3% failure Sprayer 15 m AZ500 0.8 kg/ha Premazin 2.0 kg/ha Auto-guided hoe, 2 m Girovator (medium), 2 m

STEP planter, Salix Maskiner, 4 rows Manual, 3% failure Sprayer 24 m AZ500 0.8 kg/ha Premazin 2.0 kg/ha Auto-guided hoe, 6.85 m Flail mower (Flailmaster), 2.1 m

Sprayer 15 m AZ500 0.8 kg/ha Premazin 2.0 kg/ha Broadcaster, 15 m range 120 kgN/ha, cycle 46 kgP2O5/ha, cycle 84 kgK2O/ha, cycle

Sprayer 24 m AZ500 0.8 kg/ha Premazin 2.0 kg/ha Broadcaster, 24 m range 150 kgN/ha 58 kgP2O5/ha, cycle 105 kgK2O/ha, cycle

Sprayer 15 m AZ500 0.8 kg/ha Premazin 2.0 kg/ha Broadcaster, 15 m range 180 kgN/ha, cycle 69 kgP2O5/ha, cycle 126 kgK2O/ha, cycle

Sprayer 24 m AZ500 0.8 kg/ha Premazin 2.0 kg/ha Broadcaster, 24 m range 225 kgN/ha 86 kgP2O5/ha, cycle 157 kgK2O/ha, cycle

Broadcaster, 15 m range 102 kgN/ha, cycle 39 kgP2O5/ha, cycle 71 kgK2O/ha, cycle

Crop maintenance for the 7 following cycles Weed control Fertilisation

Harvest Harvest Chipping Grub up Wood fuel processing Local transport Storage and drying Regional transport

Scenario ``high''

82 kW (110 hp)

Crop maintenance for the ®rst cycle Weed control Fertilisation

Scenario ``medium''

60 kW (80 hp)

Broadcaster, 15 m range 153 kgN/ha, cycle 59 kgP2O5/ha, cycle 107 kgK2O/ha, cycle

Stick harvester (Rodster S.M.) Chips harvester, tractor Forage harvester (Claas 695), mounted (Bender, S.M.) self-propelled Tractor-mounted chipper Diam. 25 cm Rotavator (Mericrusher, 2.3 m) Rotavator (Mericrusher, 2.3 m) Rotavator (Mericrusher, 2.3 m) Tractor & trailer 30 m3 Transport distance 5 km Storage in sticks on ®eld, natural ventilation Bulkcarrier 55 m3, transport distance 30 km

Tractor & trailer 30 m3 Transport distance 5 km Storage in chips at the farm, forced ventilation Bulkcarrier 55 m3, transport distance 30 km

Tractor & trailer 30 m3 Transport distance 5 km Storage in chips at the farm, forced ventilation Bulkcarrier 55 m3, transport distance 30 km

382

XAVIER DUBUISSON and IVAN SINTZOFF

2.2. Calculation of energy and carbon costs We calculated the energy and carbon costs of all the activities of wood fuel production.3 On the basis of the activities inventory, we have quanti®ed: Ðthe amount of diesel and lubricating oil, or electricity, consumed to carry out each operation; Ðthe amount of materials engaged in wood fuel production (fertilisers, fence, herbicides, etc.); Ðthe extent of each piece of machinery use, and wear and tear, during its utilisation in each operation. We then estimated the direct and indirect energy and carbon costs of each operation. Direct energy costs are incurred by the combustion of diesel or lubricating oil, as well as electricity consumption. We also took into consideration indirect energy costs related to the production and supply of the fuel and electricity consumed, as well as those related to materials consumption. Part of the energy cost of producing and supplying the machinery used has to be apportioned to each operation as indirect energy costs. According to a widely accepted convention for energy analysis, we did not take into account solar energy and human labour as non-renewable energy sources.3 We used the same approach to estimate the direct and indirect carbon emissions of wood fuel production. Table 2 presents the total energy costs and carbon emissions per activity. We considered a total lifetime of the plantation equal to 25 years5 to calculate the total energy cost and carbon emission of SRC cultivation.

2.3. Energy ratio, energy and carbon requirement The energy ratio is the bene®t±cost ratio in energy terms for the wood fuel production system.3 An energy ratio higher than one indicates that the wood fuel production system has a positive energy yield and is bene®cial for substituting fossil fuels. The energy bene®t provided by the wood fuel produced is calculated taking into account the moisture content at each stage of its production and transformation.3 When calculating the energy bene®t after storage and drying, we also took into account a dry matter loss due to respiratory self-heating. The net energy yield is the di€erence between the energy bene®t and energy cost per unit of land. The percentage energy pro®t is the net energy yield expressed as a percentage of the energy bene®t. The energy requirement is the total energy necessary to produce one unit of wood fuel. The carbon emission factor (CEF) is the total carbon emitted (kgC) for the production of one unit of wood energy produced (GJ). Energy ratios, energy requirements and CEF were calculated at di€erent stage of the wood fuel production: Ðafter harvest, i.e. when the wood fuel is in the form of chips at the border of the ®eld; Ðafter local transport, i.e. at the farm before storage and drying; Ðafter storage and drying at the farm; Ðafter regional transport, i.e. when the wood fuel is at the centralised conversion unit. Table 3 presents the results produced by the model for the scenario ``low'', ``medium'' and ``high''. The energy ratio obtained shows that SRC is highly bene®cial as a source of renewable energy as we obtain energy ratio between 22 and 28 after regional transport. One ha of

Table 2. Total energy costs and carbon emissions per activity GJ haÿ1 Ground preparation Crop establishment Crop maintenance Harvest and chip Grub up Total cultivation Local transport Storage and drying Regional transport TOTAL

1.39 4.44 79 95 4.10 184.94 9.77 0 48.57 243

Low

kgC haÿ1

Medium GJ haÿ1 kgC haÿ1

GJ haÿ1

47 190 1767 3151 136 5291 193 0 1645 7129

27.33 5.81 99 46 4.10 171.72 11.08 20.01 51.25 254

24.37 5.71 114 38 4.19 186.32 16.89 25.01 64.06 292

3698 238 2307 1200 72 7579 226 1749 1729 11283

High

kgC haÿ1

1007 234 2630 1322 140 5335 350 2186 2161 10032

Generation routes using wood fule

383

Table 3. Energy ratio, energy requirements and carbon requirements of wood fuel production Units Total usable wood harvested Total wood losses in storage & drying Total energy bene®t after harvest After harvest Energy ratio Percentage energy pro®t Energy requirement Carbon emission After local transport Energy ratio Percentage energy pro®t Energy requirement Carbon emission After storage and drying Energy ratio Percentage energy pro®t Energy requirement Carbon emission After regional transport Energy ratio Percentage energy pro®t Energy requirement Carbon emission

ÿ1

Low

Medium

High

odt ha odt haÿ1 GJ haÿ1

235 2.35 4197.5

276 16.56 4515.0

MJout/MJin % GJ odtÿ1 kgC GJÿ1

22.7 95.6 0.8 1.3

26.3 96.2 0.6 1.7

30.3 96.7 0.5 0.95

MJout/MJin % GJ odtÿ1 kgC GJÿ1

21.6 95.4 0.8 1.3

24.7 96.0 0.7 1.7

27.8 96.4 0.6 1.0

MJout/MJin % GJ odtÿ1 kgC GJÿ1

21.5 95.3 0.8 1.3

23 95.7 0.8 2.3

25.6 96.1 0.7 1.5

MJout/MJin % GJ odtÿ1 kgC GJÿ1

17.2 94.2 1.1 1.7

18.4 94.6 1.0 2.7

20.0 95 0.9 1.9

SRC produces a net energy yield of 3946 GJ in the scenario ``low'' to 5362 GJ in the scenario ``high''. The results of our study are in line with other studies using the same approach.1,11 However, these results can vary relatively widely according to the basic assumptions made (e.g. for yields) and management practices chosen. The scenario ``high'' is the best option for energy production even if its total energy costs are higher. This can be explained by the fact that better crop management practices result in higher yields, and that higher energy bene®ts outweigh the higher energy costs. Net carbon emissions (NCE) of wood fuel combustion (an average of 2.1 kgC GJÿ1) from SRC are ten times lower than the total carbon emissions factor of diesel oil (20 kgC GJÿ1). The scenario ``low'' has the lowest NCE after storage and drying (1.7 kgC GJÿ1) and the scenario ``medium'' the highest (2.7 kgC GJÿ1). However the NCE after harvest is the lowest in the scenario ``high''. This is due to the relatively high indirect carbon emissions due to electricity consumption for drying the wood chips by forced ventilation in the scenario ``medium'' and ``high''. The large carbon emissions due to the generation of electricity with fossil fuels also explain the highest NCE of the scenario ``medium'' where the fence implanted is electri®ed.

345 20.7 5643.8

2.4. Breakdown of energy and carbon costs The breakdown of the total energy costs between activities, from ground preparation to regional transport, shows that crop maintenance, harvest and regional transport are the most energy demanding operations. The relatively high energy cost of crop maintenance (30% of total energy costs) in the three scenarios is due mainly to the very high energy costs of fertilisers (e.g. 1 kg of nitrogen = 57.5 MJ), as well as to the implantation of a fence in the scenario ``medium'' and ``high''. Harvest and chipping of the wood fuel is also a costly operation in carbon and energy, especially for the scenario ``low'' where they are carried out in two distinct operations. Chipping naturally dried SRC sticks with a tractor-mounted chipper is a slow operation (13 hr haÿ1) requiring a continually high utilisation rate of the engine power. Total energy costs of SRC cultivation are more or less equal in the scenario ``low'' and ``high'' because the higher fuel consumption in the scenario ``low'' compensates somehow for a larger amount of fertiliser input in the scenario ``high''. The energy cost of regional transport is also relatively high (20 to 25%) and SRC cultivation should be concentrated around the energy generation unit. In the scenario ``medium'', ground preparation accounts for 33% of the total carbon

384

XAVIER DUBUISSON and IVAN SINTZOFF Table 4. Breakdown of carbon and energy costs between activities and between direct and indirect costs Low (%)

Breakdown between activities Ground preparation 1 Crop establishment 2 Crop maintenance 32 Harvest and Chip 40 Grub up 2 Local transport 4 Storage and Drying 0 Regional transport 20 Breakdown between direct and indirect costs Direct fuel costs 55 Indirect fuel costs 10 Indirect material costs 33 Indirect machinery costs 2

Energy costs Medium (%)

High (%)

Low (%)

Carbon emissions Medium (%) High (%)

11 2 41 15 2 5 8 21

10 2 47 16 2 7 10 26

1 3 25 44 2 3 0 23

33 2 20 11 1 2 16 15

10 2 26 13 1 3 22 22

40 12 45 3

37 13 46 4

35 35 26 4

14 56 27 3

19 40 36 5

emissions due to the carbon cost of manufacturing the fence, and above all, to produce the electricity necessary to power it. Electricity consumption for powering the ventilators to dry the chips also accounts for the high indirect carbon costs in the scenario ``medium'' and ``high''. Fuel and electricity consumption amounts to 50±65% of the total energy costs.Table 4 2.5. Sensitivity analysis By varying the value of certain input data by 20% over and above their reference value, we were able to measure the in¯uence of these data over the ®nal results of the model. Using this method, we tested the in¯uence of the level of yields, fertilisers, regional transport distance, and implantation of a fence, on the energy performances of the system. We also tested the in¯uence of varying the values of the conversion factors used. We measured

their in¯uence by looking at the variation of: (1) the energy ratio after harvest; and (2) the energy requirement of 1 odkg wood fuel after regional transport to a central conversion plant. Table 5 presents the results of the sensitivity analysis for the scenario ``low'', ``medium'' and ``high''. Looking at the wood fuel production factors, these results show that the level of yields and fertiliser input have a large in¯uence on the energy performances. The yields of SRC can vary signi®cantly with soil and climate conditions, as well as with cultivation practices. It is therefore necessary to be careful when generalising our results to other conditions. In order to give an indication of the impact of replacing chemical fertilisers by organic fertilisation, we introduced a value zero for fertilisers in the model, keeping other values constant. We obtained energy ratios ranging

Table 5. Results of the sensitivity analysis Scenarios Yields E.Rat. after harvest E.Req. after regional transport Fertilisers E.Rat. after harvest E.Req. after regional transport Fence E.Rat. after harvest E.Req. after regional transport Long transport distance E.Rat. after harvest E.Req. after regional transport Conversion factors* E.Rat. after harvest E.Req. after regional transport

Low (%) +20 +10$ ÿ7 +20 ÿ8 6 +20 0 0 +20 0 +4 +20 ÿ18 +22

ÿ20 ÿ12 +10 ÿ20 +9 ÿ6 no fence 0 0 ÿ20 0 ÿ4 ÿ20 +27 ÿ22

Medium (%) +20 +8 ÿ5 +20 ÿ10 8 +20 ÿ2 1 +20 0 +4 +20 ÿ15 +19

ÿ20 ÿ10 +8 ÿ20 +12 ÿ7 no fence +11 ÿ7 ÿ20 0 ÿ4 ÿ20 +21 ÿ18

High (%) +20 +7 ÿ4 +20 ÿ10 8 +20 ÿ2 2 +20 0 +4 +20 ÿ16 +20

ÿ20 ÿ9 +7 ÿ20 +13 ÿ7 no fence +13 ÿ8 ÿ20 0 ÿ5 ÿ20 +23 ÿ18

$In this case, the energy ratio after harvest of the scenario ``low'' increases by 10% over its reference value when the yields are augmented by 20%. *``Conversion factors'' refer to the coecients used to convert physical quantities of fuels, materials and machinery in energy and carbon costs: energy requirements, calori®c value, carbon content, carbon requirements, etc

Generation routes using wood fule

385

Fig. 1. Local peak power generation system.

from 40 for the scenario ``low'' to 71 for the scenario ``high''. We note that the choice of implanting a fence has a signi®cant impact on the energy ratio of the wood fuel due to the high energy cost of a fence. The sensitivity analysis also shows that the model results are highly sensitive to the conversion factors used (calori®c value of fuel, energy requirement of fuel, materials and machinery, carbon emitted by fuel and carbon requirement). 3. WOOD ENERGY CONVERSION

The objective of this part of the study was to assess the impact of generating power from SRC wood fuel compared with classical fossil fuels technologies. We evaluated the reduction of CO2 emissions by substituting fossil fuels with wood fuels. Despite the large variety of existing woodenergy technologies, we only considered three power generation technologies: Ðpeak power generation (small scale downdraft gasi®er and diesel-gas engine); Ðcogeneration of heat and power (small scale downdraft gasi®er and CHP gas engine); Ðpower generation by co®ring of wood and coal (classical pulverised coal plant). These technologies have been chosen for their potential technical and economic feasibility in Belgium in the short or medium term. 3.1. Description of the energy conversion technologies studied 3.1.1. Local peak power generation. This small scale conversion technology is designed to supply peak power to the electrical grid by

wood gasi®cation and diesel-gas combustion in an internal engine. The process is divided in 4 steps (Fig. 1). First, wood chips or chunks are dried up to 10% moisture content by recycling engine exhaust gases. Second, wood is gasi®ed in a ®xed bed downdraft gasi®er. Third, particles, volatile compounds and tars are eliminated from the gases by cycloning and wet scrubbing, and fourth, fuel gases are injected with 8% of diesel in the diesel-gas engine that drives the alternator. This unit is running in our laboratory (GAZEL System25) and will be tested in real conditions on a Belgian farm producing SRC wood fuel. The system will generate peak electricity 1500 hours per year. 3.1.2. Local heat & power cogeneration (CHP). The second conversion unit is also based on wood fuel gasi®cation (Fig. 2) to produce base power and heat 4500 hours a year. We consider a gas engine that allows to burn wood gases without any fossil fuel addition. In some situations, natural gas can be fed to the engine (on average 1% of the total energy input). Heat losses from the engine are recovered to produce hot water (<1008C). The residual heat losses are used to dry wood chips till their moisture content is <20%. This system is only suitable where there is a local demand for heat. 3.1.3. Centralised wood & coal co®ring thermal power plant. The third conversion plant analysed is a classical thermal power plant producing base power 4265 hours per year (Fig. 3). The wood fuel can replace a small percentage of pulverised coal without any boiler transformations. However, wood fuel must be pre-treated in dried wood ®nes of 1±3 mm.

Fig. 2. Local CHP system.

386

XAVIER DUBUISSON and IVAN SINTZOFF

Fig. 3. Centralised base power generation system.

Then, coal and wood co®ring generates steam to feed a classical power generation cycle. 3.1.4. Conventional generation technologies used for comparison. The three wood energy technologies chosen were compared with current conventional fossil energy systems, looking at Belgian conditions. Local peak electricity generation by gasi®cation was compared with peak electricity generation by a jetoil turbine. Local cogeneration by gasi®cation was compared with a recent CHP plant using natural gas or to the co-utilisation of heat from a gas boiler and power from the electricity network (produced by Belgian fossil fuels power plants). Centralised generation by co®ring was compared with a classical Belgian pulverised coal power plant. 3.2. Final energy conversion eciencies For each conversion system, the ®nal energy conversion eciency is calculated by dividing the ®nal usable energy output by all energy inputs (Fig. 4). The total energy input includes the direct energy content of fuels, the indirect energy embodied in fuels, and other indirect energy inputs. We simpli®ed the system analysis by only taking into account the indirect energy costs incurred by the construction of the conversion plant, and we neglected other indirect energy costs as water supply, limestone consumption, human transport, etc. The indirect energy costs have been estimated at 3 to 4 GJ per kWe of installed power production. Direct energy inputs are wood fuel inputs and other fossil fuels consumed to maintain

the process. The main part of the total indirect energy inputs in the conversion system consists in the energy embodied in the fuels burnt (Table 6). In addition to the energy embodied in wood fuel, we also considered energy costs of ®nal wood fuel pre-treatment at the generation plant. Final drying must be carried out to obtain a wood moisture content lower than 20%. This can be achieved by recovering the exhaust heat. In the case of co®ring, ®nal sizing is a very energy-intensive operation, consuming from 0.4 to 3 GJ odtÿ1.8 Estimates of the energy cost of fossil fuels production, transport, re®ning and distribution have been produced since the early seventies. However ®nal ®gures found in the literature vary widely depending on the methodology applied. It is therefore essential to establish international standards for energy accounting in order to reduce the large uncertainties remaining in the calculation of the energy embodied in fuels. Table 7 presents overall eciencies of fossil and SRC fuels production cycle. Fuel production cycle eciency expresses the energy eciency of the wood fuel production and pre-treatment (®nal sizing include). Values related to fossil fuels are presented with the lower and higher estimates found in the literature.10,13,14,17±21 In general, higher values were found in studies ignoring part of the fuel cycle. Our calculation of liquid fossil fuels energy costs is based on the European overall energy losses in the petroleum sector for transformation and distribution (85 Mtoe in 19959). To this amount, we added the energy necess-

Fig. 4. Final energy production eciency.

Generation routes using wood fule

387

Table 6. Energy costs of the materials for the construction of the conversion plants Wood consumption odt yrÿ1 Local peak power Local CHP Centralised co®ring

174 626 7030

Wood fuel (%) 92 99 1

ary for crude oil extraction and transport. Table 7 also summarises the total wood fuel production costs at the conversion unit and the wood fuel production cycles eciencies. Compared with classical fossil fuels production, SRC cultivation is an ecient system to produce fuel. Particularly, local use of SRC wood fuel appears to be very ecient. We have estimated the energy embodied in construction materials for the conversion units from our own data (for the peak power and CHP plants) and from the literature10 (for the classical coal plant). In all cases, construction energy costs are less than 1% of the overall energy consumption (Table 8). For centralised co®ring, because the plant lifetime is twice the local plant's lifetime, the energy consumed in building the plant appears negligible. We have simpli®ed the energy system analysis by ignoring other indirect energy costs. By aggregating direct and indirect energy costs, we evaluated the total energy eciencies of each wood energy route, from fuel production to energy production (Table 9). From the thermodynamical point of view, the best energy route is to produce power and heat in the same system (CHP). The di€erence between the three wood fuel production scenarios is attenuated when looking at the whole fuel cycle eciency. This is due to the fact that the energy cost of wood fuel production is a small proportion of the total energy inputs compared with the energy content of the wood Table 7. Whole fossil and SRC fuels production cycles eciencies and carbon costs

Re®ned liquid fuel from crude oil Natural gas from gas reservoirs Coal from coal seams SRC wood chips at the farm SRC wood chips after 30 km transport

Zfuel %

kgC/GJfuel

84 26*

421*

88 21*

321*

94 23* 94 21$ 86 21$

221* 120.4$ 620.4$

*Intervals express the incertitude found in the literature. $Intervals express the variance caused by cultivation practices choices

Liquid fossil (%) 8 ± ±

Gas fossil (%)

Solid fossil (%)

± 1 19

± ± 80

fuel (Table 8). Compared with oil-®red gas turbines, peak power generation from wood fuel has a better overall energy eciency. This is mainly the result of the lower energy cost of producing and supplying wood fuel opposed to jet-oil. For the centralised pulverised coal route, the diminution of the cycle production eciency is mainly caused by the higher energy consumed for wood pre-treatment (®nal crushing). It is interesting to note that the only wood energy chain that improves the global electricity production in Belgium will be local production of peak electricity.

3.3. Final carbon emissions balances Because of the neutral carbon balance of wood, carbon emissions released by the woodenergy system are limited to the net direct and indirect carbon emissions from fossil fuel and materials consumption. The calculation of direct and indirect carbon emissions incurred by the wood fuel production were carried out in the ®rst part of this study. The next step was to calculate the carbon costs of the wood fuel pre-treatment and ®nal conversion. We also evaluated carbon emissions that are avoided by the substitution of classical fossil energy by wood energy. 3.3.1. Direct carbon emissions. Table 10 presents the direct carbon emissions expressed in kilograms of fossil carbon released by GJ of ®nal energy produced. The comparison with carbon emissions from classical fossil fuels systems shows the importance of fossil fuels substitution by wood fuel. 3.3.2. Indirect carbon emissions. The carbon emissions of wood fuel production and supply are presented in Table 7 where they are compared with the carbon costs of producing and supplying fossil fuels. Carbon costs are expressed as a function of the energy content of the fuel. As we consider that the ®nal drying of wood fuel is free of energy costs (heat can be recovered at the conversion plant), the energy content of wood is 18.6 GJ odtÿ1.

388

XAVIER DUBUISSON and IVAN SINTZOFF Table 8. Fuels consumption in energy conversion systems Direct costs %

Fuel energy

Fuel production

Construction

94.4 95.1 93.9

4.8 4.6 6.1

0.8 0.3 0.0

Local peak power Local CHP Centralised co®ring

3.3.3. Avoided carbon releases in the atmosphere. In Table 10, the total carbon emissions are expressed per unit of ®nal energy output. From these estimates, we can evaluate the impact of the various wood energy routes on global change. To maximise the carbon emission reduction, it is preferable to implement new local cogeneration system, avoiding annually more than 22 tons of carbon dioxide per hectare of SRC. When there is no local heat demand, small scale peak power generation should be preferred to centralised generation. In fact, centralised co®ring avoids about 5% less carbon emissions than local systems. This ®gure results mainly from the energy costs of ®nal preparation (wood fuel sizing to 1±3 mm) for fuel pulverisation. The co®ring energy system has a better overall energy conversion eciency than the peak power system due to its high power generation eciency. But this does not lead to the highest carbon emissions reduction because of the consumption of diesel and electricity for intensive wood transport and preparation. 4. CONCLUSIONS AND RECOMMENDATIONS

Concerning the analysis of wood fuel production systems: 1. Inevitably, we had to make some assumptions when establishing and running our Table 9. Final energy eciencies of wood and fossil energy routes GJout GJÿ1 in Peak power production (wood gasi®cation/jet-oil turbine) Local cogeneration (wood gasi®cation/best available natural gas) Local heat generation (gas boiler) & centralised fossil power production Centralised co®ring

Wood energy route (%)

fossil route (%)

25 21$

2322*

61 21$

7821* 6024

33 21$

Indirect costs

3521*

*Intervals express the incertitude found in the literature. $Intervals express the variance caused by agricultural practices choices

model. Yields and energy costs can vary widely with cultivation practices and wood fuel processing techniques. There is a lack of information to demonstrate the in¯uence of fertiliser input on the yields of short rotation coppice. There remain also some uncertainties in the estimation of energy requirements of fuels, materials and machinery. It is therefore necessary to be careful when using our results for other conditions. 2. Short rotation coppice is a highly bene®cial crop as a source of renewable energy with energy ratios of 22, 23 and 28 for the scenarios ``low'', ``medium'' and ``high'' respectively. A rapid survey of the literature3,11±13 on energy crops show that short rotation coppice is well positioned among the best energy crops. For example, we found energy ratios of 22 for miscanthus, 12 for sweet sorghum, 11 for Jerusalem artichoke. However these results have to be compared with care as energy ratios can vary a lot depending on the production processes and transformation stages of biofuels, as well as on which energy costs are included. It is therefore recommended to compare energy crops on the basis of ®nal energy eciencies looking at similar energy conversion systems, before making a de®nite statement on the value of an energy crop. 3. We have seen that the scenario ``high'' gives the best results in terms of energy performance of the wood fuel production system. The energy ratio of the scenario ``high'' is about 20% higher than the scenario ``low'', after storage and drying. This is a direct e€ect of the higher yields of the scenario ``high'' due to better crop husbandry practices, although the relationship between yields and crop husbandry practices will have to be con®rmed with longer term experience. 4. Nevertheless, high energy ratios should not encourage wasteful inputs to the crop. Indeed, the level of chemical fertilisation has a signi®cant impact on the energy and

Generation routes using wood fule

389

Table 10. Final carbon emission factors and avoided carbon emissions factors CO2 emission factor kgC GJÿ1 out

Local peak power generation (wood/jet-oil) Local cogeneration (wood/natural gas) Local heat production (gas boiler) & centralised fossil power production Centralised co®ring (wood/coal)

Wood energy

Fossil energy

Avoided CO2 emissions factor 103 kgC haÿ1 yrÿ1

7.220.6$ 2.920.3$

8826* 1921*

4.620.8$ 7.521.5$

12.020.4$

4425* 7423*

2.120.8$ 4.320.8$

*Intervals express the incertitude found in the literature. $Intervals express the variance caused by agricultural practices choices

carbon costs of wood fuel production. We have seen that crop maintenance costs (mainly due to fertiliser consumption) account for around 40% of total energy costs and 25% of total carbon costs. On the other hand, if we set fertiliser input to zero, we have seen that yields could be reduced by 40% (scenario ``low'') to 60% (scenario ``high'') before the energy ratio becomes lower than the ratio of reference. Such considerations should also encourage the replacement of chemical fertilisation with organic fertilisation, using livestock e‚uent, wastewater, sludge, etc. 5. Technical improvement of the SRC cultivation machinery in the near future will have a signi®cant positive impact on the energy performances of the system. For example, if we could reduce by 50% the duration of chipping in the scenario ``low'', we could obtain an energy ratio similar to the scenario ``high'' energy ratio after harvest. Our results also con®rm that a higher level mechanisation and the use of more ecient machines result in a lower fuel consumption. All the operations or materials input involving ``fossil'' electricity consumption (like forced ventilation or fence electri®cation) should be avoided due to the high indirect carbon costs. 6. Our analysis also con®rms that energy and carbon costs of transport account for a signi®cant part of the total costs (around 20%). However they can increase up to 160 km before the energy ratio of the scenario ``high'' is halved. 7. Further research is needed to evaluate the impact of other greenhouse gas emissions caused by wood fuel production. Concerning the analysis of wood fuel conversion systems:

1. The analysis of overall energy eciencies and avoided carbon emissions of di€erent SRC wood energy systems has been completed for the whole route from cultivation to ®nal energy production. 2. New local cogeneration by gasi®cation of SRC wood fuel is the best option to maximise the reduction of atmospheric carbon emissions, with the condition that there is sucient local demand for heat. Because present use of local cogeneration is very litle developed in Belgium, we can consider that new CHP plants will be installed in the future for replacing present co-utilisation of local heat and electricity from the network. 3. Local peak power generation by gasi®cation of wood fuel is the best option if we want to maximise the global eciency of the energy system. Indeed the peak power generation route studied has a higher overall energy eciency than its counterpart using fossil fuels (oil extraction and turbojet turbines), while local cogeneration and co®ring fed by wood have lower energy eciencies than fossil systems. This can be explained by the high energy cost of preconversion treatments of wood chips (®nal drying, sizing or gasifying). 4. Uncertainties remain in our analysis due to the lack of comprehensive accounting of real energy costs of industrial activities which would allow more precise estimation of the actual energy costs of fuels, machinery, materials. Standards have to be established for the calculation of energy and carbon direct and indirect costs of fossil fuels. 5. The methodology applied here will be applied to other wood energy systems to have a global approach of the sector. In the framework of its 1996±2000 programme, the Belgian Federal Oce for Scienti®c, Technical and Cultural A€airs has

390

XAVIER DUBUISSON and IVAN SINTZOFF

launched the Woodsustain project in order to assess di€erent systems that will assure a sustainable development of the wood energy sector in Belgium.

13. 14.

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